I just recently finished a course to become a certified personal trainer. It's simply to further my knowledge of exercise and kinesiology, and to be able to legally recommend exercises to my patients when needed. (I'm currently a muscular therapist.)
Which is all very irrelevant. The point is, that while I was taking the test, the typos and grammatical errors were jumping off the page and slapping me in the face. That stuff drives me insane! It makes my eye twitch to get professional material with those kinds of errors. I'm sure I make them all the time - it's actually much harder to pick them up in your own words than it is in someone else's. But then, I don't put out professional work material without having someone else proof it! (Blog not included, so don't point out my mistakes there - I'm sure there are plenty.)
Just a few examples of what I saw: then vs. than; "or" was spelled "ore"; your vs. you're; and randomly capitalized letters for no apparent reason.
None of these are a big deal, they are not life or death, and I did not fail to get the point that was being made.
But what I've come to realize is that I think I made the wrong career choice! I should have been an editor! Getting paid to find these kinds of errors? It would be like an OCD dream come true. Puuuure satisfaction.